1. Field of the Invention
The present invention relates to switched power supplies, and more specifically, to a forward converter that incorporates synchronous rectifiers in the output stage to reduce the voltage drop and power loss associated with the use of Schottky diodes or other conventional rectifying elements. The inventive forward converter includes a drive signal generating circuit for the synchronous rectifiers which provides properly timed square wave gate drives to control the operation of the rectifiers, thereby reducing turn on and turn off losses of the rectifiers and increasing the efficiency of the converter circuit.
2. Description of the Prior Art
Switching or "switch mode" power supplies use a semiconductor device as a power switch to control the application of a voltage to a load. A forward converter is used to provide a regulated output (or load) voltage (V.sub.OUT) which is lower than the input voltage (V.sub.IN) supplied by the input power supply. FIG. 1 is a schematic diagram showing a circuit for a conventional prior art forward converter 100. Applying a waveform to gate node 103 controls the operation of power switch Q1 102 (which is shown as a MOSFET device but may be of other types). The waveform applied to gate node 103 is typically provided by a control circuit (not shown) which supplies a pulsed control signal using pulse width modulation (PWM), for example. When switch 102 is turned "on", i.e., conducting, the input voltage, V.sub.IN, provided across input supply nodes 300 and 302, is applied across the primary winding of power transformer 104. A secondary voltage, V.sub.S, is developed across the secondary winding of transformer 104 and applied across forward output rectifier D.sub.O1 106 (which then becomes forward biased). Current and power flows to output inductor L.sub.O 108 and output capacitor C.sub.O 110 (which form a LC filter), and load R.sub.L. Assuming a sufficiently large enough value for output capacitor C.sub.O 110, and neglecting diode drops and losses, the voltage across inductor 108 will be equal to V.sub.S minus the output voltage, V.sub.OUT (where V.sub.S is equal to the turns ratio of the power transformer times the input voltage, V.sub.IN, and the output voltage is seen across output nodes V.sub.OUT1 and V.sub.OUT2). The current (i.sub.L) in inductor 108 will increase linearly with time and will be described by: EQU di.sub.L /dt=(V.sub.S -V.sub.OUT)/L.sub.O.
When power switch Q1 is turned off, i.e., non-conducting, the secondary voltage V.sub.S will reverse. However, the current in inductor 108 will continue to flow in the forward direction rendering "freewheeling" output diode D.sub.O2 112 conductive (forward biased). This permits the current to continue circulating in the circuit loop formed from diode 112, inductor 108, capacitor 110, and load R.sub.L (which is applied across the output nodes). The voltage across inductor 108 eventually reverses, having a value equal to the output voltage V.sub.OUT (again neglecting diode drops). The current in inductor 108 now decreases with time, and may be described by: EQU di.sub.L /dt=(-V.sub.OUT)/L.sub.O.
In a steady-state condition, the volt-seconds applied to inductor 108 are equal in the forward and reverse directions. Thus, when the "on" period for switch 102 (t.sub.on) during a cycle is equal to the "off" period (t.sub.off) during a cycle, the output voltage V.sub.OUT will be equal to one-half the value of the secondary side voltage V.sub.S. When the ratio of the power switch's "on" time to "off" time differs from a 50% duty factor (where the duty factor is defined as t.sub.on /(t.sub.on+t.sub.off)), the output voltage is given by: EQU V.sub.OUT =V.sub.S *t.sub.on /(t.sub.on +t.sub.off).
A drawback of switch mode power circuits as described above is that in certain circumstances the output stage rectifiers (i.e., diodes 106 and 112 in FIG. 1) can be a significant source of power loss. This is particulary true in switching power supplies which are intended to produce relatively low output voltages, e.g., in the range of 2 to 3 volts. Even Schottky diodes, which are used in some conventional low output voltage supplies, have a significant voltage drop across the devices and hence can result in a relatively large power loss in such applications.
It has been suggested to use synchronous rectifiers to replace the Schottky diodes or other rectifying elements in the output stage of a forward converter. Such synchronous rectifiers are conventionally implemented in the form of a low-voltage MOSFET (with a low on-state resistance) or low-voltage bipolar junction transistor (with a low on-state voltage). However, a problem arises because the operation of the synchronous rectifiers must be synchronized with each other and with the switching components of the forward converter in order to optimize the operation of the circuit and reduce power losses. This is not readily achieved with forward converters which produce non-square wave outputs from the power transformer because such outputs cause a delay in the turn on and turn off of the rectifiers and can alter the initial timing set up between the control signals for the rectifiers. In such situations there is an increase in power loss due to the increased conduction time of the rectifying device. In addition, such circuits typically require an auxiliary control circuit which increases the cost, size, and complexity of the converter.
What is desired is a forward converter circuit which incorporates synchronous rectifiers in the output stage and which includes a control circuit to generate the control signals and properly synchronize the operation of the rectifiers with the other components of the converter.